The present invention relates generally to electric motors and the management of noise produced during operation of electric motors. More specifically, the present invention relates to noise management in brushless DC (direct current) motors and in particular to shaping the signal for energizing the coils in a brushless DC motor.
The basic construction of a brushless DC motor includes a rotating permanent magnet(s) (the permanent magnet rotor) surrounded by some number of coil windings (stator coils). A current is made to flow through each of the coils in a sequential manner, thus sequentially energizing the coils. The magnetic fields created by this flow of current through the coils interacts with the magnet(s) of the rotor. Rotary motion arises by changing the current flow in the coils as the poles of the permanent magnet rotor change angular position. The process of changing the current flow is referred to as “commutation.” The period of time during which a coil or coils is energized is commonly referred to as the “commutation period.”
The speed of the motor is controlled by how much current is made to flow through the coil windings (energizing the coils). One method of energizing the coils is to apply a constant voltage across the coils for the duration of the commutation period. Another method of energizing the coils is to apply a series of pulses across the coils for the duration of the commutation period.
To achieve variations in rotational speed, the voltage level applied to the coils is varied. In one case, the voltage level of a constant voltage supply that is applied across the coils can be varied. Thus, maximum rotational speed can be obtained by energizing the coils with the maximum value Vmax of the voltage supply. A lower rotational speed (50%, say) can be achieved by energizing the coils with a voltage level of 0.5 Vmax.
A more commonly used and power-efficient approach is to energize the coils with a pulsewidth modulated (PWM) signal. Thus, the voltage Vmax of the voltage supply is applied as a series of pulses during the commutation period. The duty cycle (ON to OFF ratio) determines the speed of the motor. For example, maximum speed calls for a 100% duty cycle, namely, always ON. A speed of 75% would call for a duty cycle of about 75% where the pulse is at Vmax for 75% of the pulse period and at ground (0 volts) for the remaining 25% of the pulse period.
FIGS. 6 and 7 show the current flow through a coil during a commutation period of duration P. FIG. 6 shows the current profile when a coil is energized by a constant voltage source. It can be seen that toward the end of the commutation period there is a current spike. This current spike is due to collapsing of the magnetic field (also referred to as the back EMF, electromotive force) in the coil when the voltage is removed at the end of commutation period.
A similar effect is seen in FIG. 7 where PWM is used to energize the coils. A similar spiking effect results at the end of the commutation period when the energizing pulses are removed toward the end of the commutation period.
These spikes can create audible noise. More significantly, the spikes can be a source of EM interference (EMI). Conventional solutions include the use of circuitry to filter the spikes resulting during operation of the fan motor, or to otherwise reduce the effects of the spikes.